. Here's a fun one. It didn't happen when I was at his house, but fortunately he made a video.

. Banging of pipes at unpredictable times. Water is NOT being used when then banging occurs. As you'll see in the video, when the inlet valve to the water heater is shut, the banging stops.

What I noticed

A VERY slight bang when hot water in kitchen was turned on and off. We were trying to imitate the problem, though this may have nothing to do with it.

A VERY slight sizzling sound like a hamburger on a griddle, only fainter, when I turned the water heater on. Perhaps that is normal.

A VERY slight tapping with the same or similar rhythm shortly after the water heater was turned off. I didn't wait around forever so I don't know if it would have gotten louder.

What I know

It is a Ruud electric heater PE2M-52-2

Water pressure in town is high, 80psi or more.

He does not have a pressure reducer. Almost every house in town does.

This has not been going on forever, only the last few months.

What I checked

Drained heater to check for sediment. Some in first 5 gallons, but not much.

Dip tube is intact and the bottom, at least, looks fine.

Elements seem fine. Only slightly corroded.

My theories which I don't have much faith in.

Air in top of heater? Shouldn't it work its way out?

Change in water pressure that a pressure reducer would solve? One that handles thermal expansion also? Then again, if it really is happening after the heat is turned off, I guess that's thermal contraction.

Add a water hammer arrestor? But where is it happening, and why now?

. Thanks for your thoughts. I'm hoping he will come up with a few more clues, as now he can audibly tell when the water heater is on.

Sounds like a heat trap in the HWT, not the pipes per se. The heat traps react to small movements of water probably due to some pressure changes on the supply side to the home/apartment. There would need to be either an expansion tank or some piping that allows for enough expansion to move the ball in the heat trap.

Hammer stops could reduce it or make it worse depending on where they are located. They act like mini expansion tanks.

First, code requires pressure reduction if the incoming water pressure exceeds 80psi. You may have a closed system, and even without a PRV, may need an expansion tank. Put in both and see if it solves the mysterious sounds. Water expands when it is heated - pipes aren't very elastic, the water needs somewhere to go.

The pressure rise is pretty much linear as the water is heated. But, it may be nothing happens much until it reaches some level and that could be near when the thing is done. The amount of expansion depends on how much water needs to be heated, so it would be much more apparant after emptying it, like say for filling a big bathtub, than only a little when you were washing your hands (you'd probably never notice that).

Since water doesn't compress for practical purposes, any heating will raise the pressure of a closed system. To check if pressure is something that needs to be addressed, buy a ~$10 water pressure gauge - get one with a second 'tattle tale' hand that shows peak pressure and install it somewhere in the house (you could put it on the drain line of the WH). Leave it for 24 hours or so and check the instantaneous pressure occassionally, and note the peak pressure. If the peak exceeds 80psi, code requires a PRV, and that calls for an expansion tank as well. Resolve that, and see if your problem goes away, but if it's over 80psi, you should do this anyway.

Water will try to escape whatever way it can as it expands in the WH. But, regardless of that, you should check the static and dynamic pressures, if they exceed 80psi on a regular basis, you need an expansion tank and a prv.

I once encountered a situation where the neighbor's pressure regulator was generating pulses upstream that telegraphed into his home and would rattle his pipes. He did not have a regulator. I suggested he install a small expansion tank were the water line entered.

80 psi is too high whether it is the cause of the banging or not. You need a PRV and a thermal expansion tank. 60 psi is ample pressure and will be much easier on washer, toilet, and dishwasher valves.